In exhaust gases which are emitted from internal combustion engines, such as diesel engines, particulates are included. In the particulates, substances which are harmful to human bodies are included, and it has become an environmental assignment to remove them.
For removing the particulates, filter catalysts have been used. Filter catalysts are set forth in Japanese Unexamined Patent Publication (KOKAI) No. 9-173,866, Japanese Unexamined Patent Publication (KOKAI) No. 9-220,423, Japanese Unexamined Patent Publication (KOKAI) No. 9-276,708 and Japanese Unexamined Patent Publication (KOKAI) No. 2002-295,228, for example. A conventional filter catalyst has a construction in which a catalytic layer comprising a loading layer composed of alumina, and the like, and a catalytic metal loaded on the loading layer is formed on a catalyst-support substrate composed of porous ceramic having chained pores. And, when exhaust gases pass the ventilation holes formed out of the chained pores of the catalyst-support substrate, the filter catalyst captures the particulates. The catalytic layer decomposes the captured particulates. At this moment, if the ventilation holes are not formed in the filter catalyst to the extent that the exhaust gases pass, the captured particulates deposit to close the ventilation holes so that the pressure loss increases when the exhaust gases pass. When the opening diameters of the ventilation holes are enlarged in order to reduce the pressure loss, it has become impossible to capture the particulates.
The conventional filter catalyst has a problem in that the catalytic layer, formed on the catalytic-support substrate, has narrowed down the opening diameters of pores so that the ventilation holes have come not to have sufficient opening diameters or the ventilation holes have come not to be opened (have been closed). Specifically, the catalytic layer of the filter catalyst is produced by preparing a slurry in which particles, such as alumina, are dispersed, coating this slurry onto a catalyst-support substrate, drying-calcining it to form a loading layer, and thereafter loading a catalytic metal thereon. During the application of the slurry onto the catalyst-support substrate, since the slurry does not penetrate deep inside the pores of the catalyst-support substrate sufficiently, it is present adjacent to the opening of the pores of the catalyst-support substrate in a deviated manner. Since the catalytic layer is formed by drying-calcining it in this state, the diametric reduction or closure of the opening of the ventilation holes occurs so that it has become impossible to form sufficient ventilation holes as a filter catalyst.